The present invention relates to a method of detecting a venous network from a portion of an individual's body for the purposes of identifying said individual. The invention also relates to a method of biometrically recognizing an individual.
The use of automatic biometric recognition methods is becoming more and more widespread for identifying individuals, e.g. in the context of border controls, controlling access to secure zones such as boarding areas in airports, etc., or controlling access to data.
A biometric recognition method generally comprises the following steps:                capturing biometric characteristics on a portion of an individual's body;        comparing the biometric characteristics with stored biometric characteristics; and        on the basis of the comparison, determining whether the individual is or is not recognized.        
A biometric recognition method is performed by means of an automatic biometric recognition system having means for capturing biometric characteristics forming a signature, means for storing biometric characteristics forming signatures, and means for comparing captured biometric characteristics with the stored biometric characteristics. The capture means are often image capture means. The memory or storage means may for example be a memory (an integrated circuit, a bar code, etc.) that is incorporated in a passport in order to store biometric characteristics of the holder of the passport, or a memory of a computer unit containing a database associating the biometric characteristics of individuals with data for identifying those individuals. The comparison means are computer units incorporating calculation means for executing algorithms for processing and comparing biometric characteristics.
The biometric characteristics of the hand, and more particularly of the fingers of individuals are used very often for performing such methods. It is thus known to perform biometric recognition on the basis of fingerprints or on the basis of venous networks of fingers. With venous networks, the capture means are constituted by an image sensor.
In the context of an automatic biometric recognition method, there is no operator present to monitor image capture, such that it is possible for a dishonest person to attempt to fool the system by presenting the capture means with a representation of the biometric characteristics of a registered person, i.e. some other person who has been properly identified and whose biometric characteristics have been stored, in order to enable that other person to be recognized by the system. The representation may be in the form of a fake finger having biometric characteristics, or it may merely be an image of a venous network.
Automatic biometric recognition systems are thus provided with means for detecting that kind of fraud. These means are generally of hardware type and they seek for example to detect such fraud by:                detecting variation in the flow of blood in the venous network as a function of pressure between the finger and the sensor; or        interferometrically analyzing a wave diffused by the finger; or indeed        by studying differences in absorption by the finger of two light beams at different wavelengths.        
Those systems require appropriate technical architectures that make them relatively expensive.
It can thus be understood that biometrically identifying an individual assumes that it is possible to solve two distinct types of problem:                a fraud-detection problem of determining whether the body portion as presented is indeed a portion of a living body; and        an identification problem of extracting from the body portion a biometric signature for comparison with stored biometric signatures.        
If a dishonest person manages to reproduce an artificial biometric signature, that may satisfy the identification problem.
It follows that whether or not the body portion is living cannot be determined solely from a successful comparison of biometric signatures.
Specifically, biometric recognition algorithms are arranged only to extract digital signatures of a medium, regardless of its nature, and to compare them with biometric signatures that have been stored. In contrast, algorithms for detecting fraud are arranged to determine the nature of the medium presented without extracting the biometric signature therefrom. This is because the characteristics that serve to reveal whether the medium is living and the characteristics that make recognition possible are not the same or are not used in the same way.